Outbound Queue Strange but Predictable Behavior

[deanstow]

We are having a very strange even concerning two domains to which we are trying to send email.

At first it seemed like intermittent inability to send. Further testing showed something strange.

If you send one email to the domain - it will stay in the Outgoing Mail queue.
If you send two emails to the domain - they will stay in the Outgoing Mail queue.
If you send three emails to the domain - they will stay in the Outgoing Mail queue.

When you send the fourth email to the domain - they will all go!

The common thread between the two domains we have observed this happening to is that they end up resolving to something ending in mail.protection.outloook.com

Has anyone else seen this behavior?

Does anyone have any ideas?

Thanking you in advance

Dean